PPARα via HNF4α regulates the expression of genes encoding hepatic amino acid catabolizing enzymes to maintain metabolic homeostasis

The liver is the main organ involved in the metabolism of amino acids (AA), which are oxidized by amino acid catabolizing enzymes (AACE). Peroxisome proliferator-activated receptor-alpha (PPAR alpha) stimulates fatty acid beta-oxidation, and there is evidence that it can modulate hepatic AA oxidation during the transition of energy fuels. To understand the role and mechanism of PPAR alpha's regulation of AA catabolism, the metabolic and molecular adaptations of Ppara-null mice were studied. The role of PPAR alpha on AA metabolism was examined by in vitro and in vivo studies. In wild-type and Ppara-null mice, fed increasing concentrations of the dietary protein/carbohydrate ratio, we measured metabolic parameters, and livers were analyzed by microarray analysis, histology and Western blot. Functional enrichment analysis, EMSA and gene reporter assays were performed. Ppara-null mice presented increased expression of AACE in liver affecting AA, lipid and carbohydrate metabolism. Ppara-null mice had increased glucagon/ insulin ratio (7.2-fold), higher serum urea (73.1 %), lower body protein content (19.7 %) and decreased several serum AA in response to a high-protein/low-carbohydrate diet. A functional network of differentially expressed genes, suggested that changes in the expression of AACE were regulated by an interrelationship between PPAR alpha and HNF4 alpha. Our data indicated that the expression of AACE is down-regulated through PPAR alpha by attenuating HNF4 alpha transcriptional activity as observed in the serine dehydratase gene promoter. PPAR alpha via HNF4 alpha maintains body protein metabolic homeostasis by down-regulating genes involved in amino acid catabolism for preserving body nitrogen.